The non-combustible contaminant nitrogen is frequently present in natural gas, often to such an extent that the natural gas cannot be utilized as a fuel due to its low energy content and decreased environmental acceptability. For example, it has been estimated that 25% of the natural gas reserves in the United States contains unacceptably high levels of nitrogen. Thus, utilization of these natural gas reserves requires treatment to remove nitrogen.
Efforts to remove nitrogen from natural gas have included methane sorption, pressure-swing adsorption and various techniques of cryogenic distillation such as liquefaction, turbocryogenic distillation, and "cold box" separation efforts. Such methods, though successful, have been relatively expensive and inefficient. Therefore, there still exists a need for a simple, efficient and low cost method of selectively removing nitrogen from natural gas.
A substantial body of literature describes the synthesis, characterization and reactivity of transition metal-nitrogen complexes. However, the focus of this work has been substantially aimed at mimicking the ability of the enzyme nitrogenase to fix, that is reduce, nitrogen, typically to ammonia or hydrazine. See, for example, Chatt et al., 78 Chem. Rev. 589 (1978) and Dilworth et al., "Reactions of Dinitrogen Promoted by Transition Metal Compounds," in 3 Comprehensive Organometallic Chemistry 1073 (1982). Hence, the work has been aimed toward either preparing stable nitrogen complexes or identification of complexes that catalyze reduction of nitrogen, and not toward reversible nitrogen binding. Examples of such stable transition metal-nitrogen complexes are as follows:
[Fe(DEPE).sub.2 (N.sub.2)(H)]BPh.sub.4 (DEPE=1,2-bis(diethylphosphino)ethane); PA1 [Fe(DIPHOS).sub.2 (N.sub.2)(H)]BPh.sub.4 (DIPHOS=1,2-bis (diphenylphosphino)ethane) PA1 [Mo(TRIPHOS)(DIPHOS)(N.sub.2)] (TRIPHOS=PhP(CH.sub.2 CH.sub.2 PPh.sub.2).sub.2); PA1 [Co(H)(N.sub.2)(PR.sub.3).sub.3 ]; and PA1 [Ru(NH.sub.3).sub.5 (N.sub.2)]Cl.sub.2 PA1 [Mo(N.sub.2).sub.2 (PPh.sub.2 Me).sub.4 ] in pyridine (Manez et al., JCS Dalton 1291 (1992)); PA1 [Mo(N.sub.2).sub.2 (DIPHOS).sub.2 ] in nitriles (Carter et al., 181 J. Organometal. Chem. 105 (1979)); PA1 [Fe(N.sub.2)(H).sub.2 (PR).sub.3 ]BPh.sub.4 +CO or CH.sub.3 CN (Aresta et al., 5 Inorg. Chimica Acta 203 (1971)); and PA1 [Ru(NH.sub.3).sub.5 N.sub.2 ]Cl.sub.2 +pyridine, NH.sub.3, dimethylsulfoxide (DMSO), Br.sup.-, I.sup.- or Cl.sup.- (Allen et al., 89 JACS 5595 (1967)). PA1 0.002 gM Ru.sup.11 (N.sub.2) (L) (TMP) in benzene (TMP=5,10,15,20-meso tetramesitylporphyrin; PA1 L=tetrahydrofuran (THF) or CH.sub.3 CN) (Camenzind et al., JCS Chem. Comm. 1137 (1986)); PA1 0.002 gM [Ru.sup.11 (C6-PBP)(1,5-DCI)] in toluene (C6-PBP=a strapped porphyrin, 1,5-DCI=1,5-dicyclohexylimidazole) (Collman et al., 110 JACS 3486 (1988)); PA1 0.07 gM Mo.sup.0 (N.sub.2) (TRIPHOS) [PMe.sub.2 Ph].sub.2 in THF (TRIPHOS=(George et al., 27 Inorg. Chem. 2909 (1988)); and PA1 0.01 gM Mo.sup.0 (CO) (N.sub.2) (DIPHOS).sub.2 in benzene (Tatsumi et al., 114 J. Organometal. Chem. C27 (1976)).
where R.sub.3 =Ph.sub.3 or Me.sub.2 Ph, Me=methyl and Ph=phenyl).
Some complexes that are known to bind molecular nitrogen desorb the molecular nitrogen through competitive displacement. However, generally these compounds cannot rebind N.sub.2 ; some examples include:
Reversible molecular nitrogen complexation has been demonstrated in the following solutions, but such solutions are not suitable for use in the present invention in that they have little or no selectivity for nitrogen over other gases such as hydrocarbons (e.g., methane and ethane) due to low solubility of the nitrogen complex in the solvent and in that methane is highly soluble in the solvents.